Shot peening method

ABSTRACT

Metal sheets are peen formed by projecting peening shot from centrifugal shot throwing wheels against the sheet passing thereunder. The centrifugal shot throwing wheels are also used for saturation peening metal objects.

United States Patent Baughman [451 Dec. 12,1972

[54] SHOT PEENING METHOD 2,881,506 4/1959 Fuchs ..72/53 2,933,802 4/1960Fuchs ..72/53 3,517,465 6/1970 Freeman ..5 1/9 3,545,996 12/1970 Duncan..5 1/319 FOREIGN PATENTS OR APPLICATIONS 459,239 12/1936 Great Britain..5 119 Primary Examiner-Richard J. Herbst Assistant Examiner-Gene P.Crosby Attorney-David E. Dougherty and Robert E. Walker [57] ABSTRACTMetal sheets are peen formed by projecting peening shot from centrifugalshot throwing wheels against the sheet passing thereunder. Thecentrifugal shot throwing wheels are also used for saturation peeningmetal objects.

12 Claims, 15 Drawing Figures PATENTEnmmm 3.705.510

sum 5 or 7 PATENTED DEC 12 1972 SHEET 6 0F 7 vQ 3KU EH sum W QQPATENTEDnEc 12 I972 SHEET 7 BF 7 mumQ .E E I! sum assume METHODBACKGROUND OF INVENTION This invention relates to peen forming metalsheets and other peening procedures and is particularly adapted for usein the aircraft industry.

Ideally, the aircraft engineer, in the design of such structures ascantilevered aircraft wings, should use one piece structures to preventthe requirement of weight adding splices. Also, such structures shouldhave constantly tapering thicknesses to carry the necessary loads withminimum structural weight. He is usually thwarted in this goal, however,by the limitations imposed by available manufacturing techniques. Forexample, aircraft skins with simple airfoil curvatures are usuallycontoured by bumping in a press brake or rolling between forming rolls.These techniques are not practical for long tapered skin sections.

Compound contoured skins, especially of heavy sections, are verydifficult to form with consistent accuracy. They are usually stretchformed over massive contoured dies. The material is in a soft conditionand is stretched through its yield point to induce permanent set at thecontour desired. The parts must later be heat treated to a hardcondition and the excess material required for the stretch operation istrimmed away and discarded. Parts of high strength materials may requirehot forming, in which case both the part and the die must be pro-heated.This method also limits the part size to the size of the stretch pressor form dies, both of which have practical size limitations. Parts thatare tapered in thickness cannot be stretch formed.

Shot peen forming overcomes these limitations in that it can form simpleor compound contours in tapered skins. Peening shot is applied directlyto material in its hard state or final temper and therefore requires nosubsequent heat treatment. Parts can be cut to a predictable undersizeand after peening will be to exact size eliminating waste of expensivematerial. Shot peen forming further requires no expensive contouredstretch dies thereby eliminating allied storage and transportationproblems. The process is versatile in that with proper automatedcontrols any contour within the capabilities of the system can berepeated accurately. Subtle application changes are possible toaccommodate known variations in the material or the process. Part lengthneed not be limited by this system. Shot peen forming is also used tocontour extruded or machined structural shapes. It is also used tostraighten parts which have been misformed by other manufacturingmethods. Another major advantage of peen forming is that the surfacelayer of compressive stresses resulting from peening can significantlyimprove the fatigue life of the parts and help prevent stress corrosioncracking.

The application of simple curvatures to aircraft skins by shot peenforming is not new. It has been done with air nozzle peening for manyyears as exemplified by US. Pat. No. 2,701,408. Methods have beendeveloped to control part deformation while peening so that compoundcurvatures could be generated. On heavier skins pre-stressing isrequired and the use of large diameter balls, three-sixteenths inch andone-fourth inch, is sometimes necessary. The feasibility of shot peenforming has long been appreciated, but its use has been restricted torelatively small structures or low production applications because ofthe problems inherent in the use of air nozzles and the requirement forlarge quantities of compressed air.

SUMMARY OF INVENTION An object of this invention is to provide a methodof shot peen forming which overcomes the disadvantages of conventionalpeen forming by use of air nozzles.

A further object of this invention is to provide such a method whichlends itself to high production rates as is necessary in massproduction.

In accordance with this invention metal sheets are peen formed byprojecting peening shot from centrifw gal shot throwing wheels againstthe sheet passing thereunder. In accordance with another aspect of thisinvention the centrifugal shot throwing wheels are also used forsaturation peening metal objects.

In an advantageous form of this invention the peening shot is projectedfrom a bank of centrifugal throwing wheels with the individual wheelsbeing mounted on a common mounting member in a staggered arrangement inthat the set of wheels on one side of the mounting member areintermediate the wheels on the other side thereof. The wheels may bedisposed across the moving sheet with the shot patterns produced by thewheels overlapping to assure complete coverage of the sheet and toeliminate the necessity for oscillating the wheels as is conventionalwith air nozzle methods.

An additional free roving wheel may be utilized for touch-up work bydisposing this additional wheel downstream or upstream from the bank ofwheels.

The same apparatus for peen forming may also be used for saturationpeening by rotating the bank of wheels and oscillating the bank in atransverse direction back and forth over a plurality of longitudinallymoving metal members such as stringers. With this particular arrangementof wheels a novel pattern is obtained whereby I00 percent coverage withthe peening shot is assured.

The concepts of this invention may also be applied to form intricateshapes such as saddle back or dihedral shapes utilized in wingstructures. Thus, for example, the air foil contour may be imparted tothe wing skin in accordance with this invention and then dihedral shapemay be imparted thereto by masking areas of the skin outside the saddleback so that only the saddle back area is exposed to the shot.

THE DRAWINGS FIG. 1 is an elevational end view partly in section of anapparatus for peen forming and saturation peening in accordance withthis invention;

FIG. 2 is a side view of the apparatus shown in FIG. 1;

FIG. 3 is a plan view showing the feed structure of the apparatus shownin FIG. 1',

FIG. 4 is an end elevational view of the apparatus shown in FIG. 1 inone phase of operation;

FIG. 5 is a plan view of the apparatus shown in FIG.

FIG. 6 is an end view similar to FIG. 4 in a different phase ofoperation;

FIG. 7 is a plan view of the apparatus shown in FIG. 6;

FIGS. 8-11 are plan views showing the peening pattern resulting from theuse of the apparatus shown in FIGS. 6-7,

FIG. 12 is a plan view illustrating the forming of a saddle back in anaircraft wing skin section;

FIG. 13 is a side view of the wing skin shown in FIG. 12; and

FIGS. 14-15 are schematic showings of the circuitry utilized with thisinvention.

DETAILED DESCRIPTION FIGS. 1-2 show the general arrangement for peenforming and saturation peening metal pieces in accordance with thisinvention. In general this arrangement 11 includes a treating chamberhaving a bank 12 of centrifugal throwing wheels 36 therein with anauxiliary wheel 14 also located in chamber 10. A work piece conveyingdevice 16 rides on tracks 17 and moves the metal pieces being treatedunder the centrifugal throwing wheels. Each end of chamber 10 includessealant doors which may be of any suitable construction such as rubberflaps 18 to prevent the peening shot from escaping from the chamber 10.

Shot is delivered from main hopper 20 into a pair of hoppers 22, 24 andultimately to the throwing wheels. Spent shot is collected in thetapered portion 26 at the bottom of the treating chamber 10 andultimately recycled back to hopper 20 through conventional separatingand elevating devices 28, 30, 32.

FIGS. 4-5 show the utilization of this invention for peen forming skinsof aircraft wings. As indicated therein and in conjunction with FIG. 1,the bank 12 of throwing wheels includes centrally mounted rotatable boom34 to which is connected four individual throwing wheels 36.Advantageously, wheels 36 are of the type illustrated and disclosed indetail in eopending application Ser. No. 687,701; filed Dec. 4, 1967,the details of which are incorporated herein by reference thereto.Accordingly, reference will not be made to the specific details of thesethrowing wheels except where necessary to facilitate an understanding ofthese wheels within the concepts of this invention. It is noted that thewheels are mounted directly on a motor and are capable of obtaininguniversal type movement so as to provide an infinite variety ofadjustments for the throwing pattern of the shot propelled by eachwheel. The wheels 36 may, for example pivot about axes 35, 37, whilesupport boom 34 may rotate, move up and down, and move back and forth.Shot is fed from bin 22 through individual conduits 38 for each of thethrowing wheels 36. Each conduit 38 terminates in a compartment 40 (FIG.3) in the compartmentalized boom 34. The shot travels down compartments40 and exits therefrom through conduits 42 into the individual throwingwheels 36. By proper adjustment it is possible to close off certain ofthe conduits so that any number of the throwing wheels 36 may berendered inactive for smaller jobs.

As later described boom 34 is connected for 90 rotation by means of anysuitable actuating member such as cylinder 44 having piston rod 46reciprocating therein and connected to bracket 48 to oscillate or rotatethe boom 34 upon movement of the piston rod 46 into and out of cylinder44. As is apparent from FIG. 3, although the boom 34 oscillates, by thearrangements of the compartments 40 wherein each compartment is arcuatewith an arc length greater than 90 each conduit 38 remains in registrywith its compartment 40 regardless of the rotational position of boom34, thereby assuring continuous feed of shot to the wheels 36.

As shown in FIG. 5 the wheels 36 are arranged in two sets in a staggeredfashion wherein each set is on a separate side on the boom or commonmounting member 34. FIG. 5 also shows the inclusion of an additionalthrowing wheel 14 downstream from the bank of wheels 12. Wheel 14 isalso of the same general construction as the type of universally mountedwheel shown and described in eopending application Ser. No. 687,701;filed Dec. 4, 1967. This wheel 14 is likewise fed from common hopper 20.For example as shown in FIG. 2 hopper 20 has two outlet pipes 50, 52 forfeeding shot into hoppers 22 and 24. The shot from hopper 24 is then fedthrough conduit 54 into wheel 14.

As shown in FIGS. 1-2 bank 12 is mounted on car 56 whereby the entirebank of wheels may move in the direction indicated by the arrow 58.Conduit 50 is so arranged as to remain in registry with hopper 22regardless of the position of its supporting car 56. Similarly, wheel 14is not only universally mounted, per se, but is also mounted on car 60for oscillating in the direction of the arrow 58. Additionally, the bankof wheels 36 and the individual wheel 14 may be moved in a verticaldirection. FIG. 1 for example shows the guide rollers 62 to assureproper vertical orientation of the bank 12. Moreover, limit switches 64are suitably arranged to control the oscillatory movement of the cars 56and 60. Thus, for example car 56 would move toward the left as shown inFIG. 1 until limit switch 64 is contacted whereupon the direction of thedriving motor would be reversed to change the direction of movement ofcar 56 to the right until a further limit switch is contacted to againreverse the direction of movement of car 56. Similar operations wouldalso take place with respect to car 60.

As shown in FIGS. 1-2 bank 12 is mounted on car 56 whereby the entirebank of wheels may move in the direction indicated by the arrow 58.Conduit 50 is so arranged as to remain in registry with hopper 22regardless of the position of its supporting car 56. Similarly, wheel I4is not only universally mounted, per se, but is also mounted on car 60for oscillating in the direction of the arrow 58. Additionally, the bankof wheels 36 and the individual wheel 14 may be moved in a verticaldirection. FIG. 1 for example shows the guide rollers 62 to assureproper vertical orientation of the bank 12. Moreover, limit switches 64are suitably arranged to control the oscillatory movement of the cars 56and 60. Thus for example car 56 would move toward the left as shown inFIG. 1 until limit switch 64 is contacted whereupon the direction of thedriving motor would be reversed to change the direction of movement ofcar 56 to the right until a further limit switch is contacted. Similaroperations would also take place with respect to car 60.

FIGS. 4-5 show the utilization of apparatus 11 of peen forming aircraftwing skin sections. As indicated therein a flat metal sheet 66 is placedon forms 68 and clamped thereto by any suitable devices such as C-clamps 70 to bend the sheet in the desired air-foil contour. n thinnersheets this pre-forming or prestressing on forms is not required, partis peened flat, and the C-clamps are not required or act only aslocators. Forms 68 are in turn mounted on an elongated car 16 fortravelling under the various centrifugal throwing wheels. As illustratedin FIGS. 4-5 free roving wheel 14 is arranged longitudinally offset fromthe bank of wheels 36 so that the combination of wheels 14 and 36 formsa plurality of peening patterns which extend completely across the sheet66. For smaller dimension sheets the bank of wheels 36 would besufficient to provide the necessary peening coverage with the shot andfor even smaller sheets certain individual wheels 36 can be inactivated.Peening shot is projected from the wheels 14 and 36 to permanentlydeform and thus provide the desired contour to sheet 66. With thisarrangement it is not necessary to impart any oscillating movement as isrequired with conventional air nozzles.

As can be appreciated the inclusion of free moving wheel 14 lends muchversatility to apparatus 11. Wheel 14 may for example be used for skinstraightening or touch-up work. This wheel can be remotely controlled tomove horizontally or transverse to the work car travel or to move up anddown. Moreover, the wheel can rotate around a vertical axis or can rollon its own horizontal axis. The wheel 14 can be used in combination withthe bank of wheels for either adding to the transverse coverage of thecombination of wheels or to alter the general curvature of the skin bylocating it directly downstream from one of the other wheels so that acertain portion of the skin will be subject to greater peening action.

FIGS. 6-7 show how the same apparatus 11 by certain adjustments can beconverted from a peen forming device to a device for saturation peening.As indicated therein the bank of wheels 36 is rotated 90 from theposition indicated in FIGS. 45 to the position indicated in FIGS. -6wherein pairs of the wheels are longitudinally aligned with respect toeach other. The saturation peening unlike peen forming is not intendedto deform the shape of the object being peened but rather is intended toimpart certain mechanical characteristics to the object. For example thesaturation peening could be utilized for stress relieving and surfaceimprovement. During most saturation peening operations utilizingapparatus 11, free moving wheel 14 would be inactivated and thesaturation peening would be accomplished by oscillating the banks ofwheels 36. Wheels 36 are driven by static variable frequency drives sothat very low intensities for saturation peening to very highfrequencies for peen forming would be possible. The small and compactwheel design makes it possible to use a mounting which allows fivemotions so that the shot stream can be zeroed in wherever it is wanted.Advantageously, larger diameter shot is used to reduce surface roughnesswhile shot loss from breakdown is almost non-existent. The shot ispreferably made by ball-bearing techniques and is very smooth, sphericaland consistent in size. Each wheel 36 can handle more than 36,000 poundsof shot per hour at shot velocities up to and exceeding 265 ft. per see.with the shot up to A inch diameter as compared with 1/16 inch maximumdiameter shot heretofore generally used for peen forming.

Ill"

The concepts of this invention may be applied for peen forming fiatskins without significant stiffening members although longitudinalstiffeners do not limit peen forming of simple air foil contours. It ispossible to free form thin sections merely by controlling the shotvelocity, volume and coverage by use of a digital or numerical controlsystem. Heavier sections, however, might require pre-stressing by theuse of holding devices such as C-clamps while the peening operationtakes place. Where free formed sections are peened it is not necessaryto utilize a mold or female die. Such free forming may be utilized toproduce aircraft skins up to h inch thick as well as mold steel up tothe same thickness. Thus the concepts of this invention could apply tothe process of making larger-heavy wall segments for welding intocylindrical or compound shapes such as pressure vessels or radarantenna.

Since centrifugal shot peening produces a somewhat rough orange peel"surface on the formed parts the surface can be blended and smoothed byusing the same shot size and pattern at lower intensities. Larger shotgives a smoother surface because the ball radius is larger and theindentation depth and ball velocity are less.

Referring again to FIGS. 5-6 the saturation peening is accomplished byplacing the objects such as longitudinal stringers 72 on the work car 16and adjusting the bank of wheels 36 to the proper height with respect tothe stringers 72. FIG. 6 for example shows how the bank of wheels 36 iscapable of vertically moving the distance B. Wheels 36 are angled to theproper position such as 45 and work car 56 (FIG. 1) is set in motion tooscillate back and forth whereby an oscillatory movement equal to thedistance A of FIG. 6 is imparted to the wheels 31. Because of theangular arrangement of the wheels the vertical outer side walls 720disposed toward wheel 360 are peened by wheel 360, while the verticalinner side walls 72b disposed toward wheel 36b are peened by wheel 36b.

As the wheels 36 oscillate back and forth above stringers 72 a novelpeening pattern is formed whereby in two complete oscillations there is100 percent saturation peening or peening coverage of a given area ofthe stringers. This pattern is illustrated in FIGS. 8-1 1. FIG. 8illustrates the patterns c, d, e and f obtained from wheels 36c, 36d,36e, and 36f when the wheels have moved once across the stringer 72which in turn moves in the direction indicated by the arrow 74. As isindicated in FIG. 8 when the bank of wheels 36 has moved once completelyacross stringer 72 five paths are thus formed with one of the paths 3being completely devoid of peening treatment. This vacant path resultsfrom the particular arrangement of staggered wheels 36.

FIG. 9 shows the resultant pattern after wheels 36 have moved completelyback and forth across stringer 72. As indicated therein triangular areas76, 78 and 80 as indicated by the overlapped hatching and stippling havebeen subjected to a double coverage. Areas 82, 84, 86 and 88, however,have only had a single application of peening, while area 90 stillremains unpeened.

FIG. 10 shows the resultant patterns when the bank of wheels 36 has madeits third pass or the beginning of its second oscillation over stringer72. As indicated therein only the four triangular areas 92, 94, 96 and98 have had a single application of shot peening while the remainder ofthe areas have had at least two applications of peening.

Referring again to FIG. 8 as noted above the area covered by wheels36c-f includes the five bands indicated as c, d, g, e and 1' when thewheels make one pass over the stringer 72. Since the stringer, however,is moving longitudinally while the wheels are oscillating or movingperpendicular thereto, when the wheels 36 make a second pass a zig-zagpattern is formed as indicated by the bands c, d, g, e, f, h and i.(Each area designated as a band is at an angle equal to the angle ofbands c, d, g, e and fof FIG. 8.) It is noted that bands h and i areonly partial areas which do not extend completely across the stringer72. These bands are then completed when wheels 36 make the third pass asindicated in FIG. 10. FIG. 11 shows the resultant pattern when thefourth pass or second oscillation is completed. As indicated therein theareas covered by the wheels 36 are designated as c, d, g, e, f, h, i, j,k although the areas j and k do not extend completely across thestringer 72. As indicated by the overlapped hatching, the area of thefive bands 3, e, f, h and i have been completely covered at least twiceby the peening wheels 36. This five-band area corresponds in dimensionto the original area c, d, g, e, and f illustrated in FIG. 8 but isshifted by two band widths because stringers 72 are moving during thepeening wheel oscillation.

FIGS. 8-11 merely show the result obtained over a limited area by twocomplete oscillations of the bank of peening wheels. Since the peeningwheels continuously oscillate while the stringers are moving, similarpatterns are obtained over the entire surface of the stringers so that acomplete saturation peening operation thereby results.

FIGS. 12-13 illustrate another aspect of this invention wherein the bank12 of wheels 36 is utilized to form a dihedral or saddle back break inan aircraft wing skin section. In this embodiment such a compoundcontour as the dihedral break frequently is aerodymically necessary foraircraft wings but has, however, presented a particular problem to theprior art. It was commonly necessary in the prior art to separately formthe skin sections on each side of the dihedral break or saddle back andalso to form a separate saddle back section. All three sections werethen secured together, which led to a number of disadvantages bothaerodynamically and with respect to the added weight caused by securingthe sections together. In accordance with this invention it is possibleto form an integral aircraft wing skin having a saddle back section.This is accomplished by first forming the air foil contour on a skinsection 100 in the manner described with respect to FIGS. 4-5. Asindicated in FIG. 12 the contoured skin section 100 is then masked inthe areas 102, 104 with the saddle back 106 being disposed between thesemasked sections. Any suitable masking material such as sheets of rubberor layers of heavy masking tape may be used. The bank of wheels 36 thenoscillates over areas 102, 104, and 106 along for example center line108 with the peening shot affecting only unmasked area 106 to result ina saddle back formation 110 between the areas 112 and 114 (FIG. 13) ofthe wing skin 100. Although FIG. 12 illustrates the bank of wheels asoscillating over the wing skin 100, for certain applications the wheelsmay be stationary.

By utilizing the techniques described with respect to FIGS. 4-5 and12-13, it is thereby possible to form a pair of mating wing skins havingthe desired aerodynamic contour including the dihedral or saddle backsections. The pair of mating skin sections may thereafter be securedtogether to form a wing which is schematically illustrated as beingsecured to the body of the airplane 116 in FIGS. 12-13.

FIGS. 14-15 schematically illustrate possible electrical controls toassure a synchronization of the transverse movement of wheels 36 withrespect to the longitudinal movement of the work car 16 to assure that auniform pattern such as illustrated in FIGS. 8-11 will be obtained.Generally these controls include a pulse generator 118 schematicallyillustrated in FIG. 2 as being suitably mounted on the work car pullerdrive which for example may be a 7 Va I-IP (SCR) DC variable speedmotor. FIG. 14 illustrates the transmitter 118 which is driven by thework car and also transformer 120 driven by the car 56 for the bank ofwheels 36. The signals generated by the longitudinal moving work car 16and the traversely moving car 56 are compared by phase detector 122 toassure a synchronization of the transverse and longitudinal movements.If the movements are not synchronized the oscillation might for examplebe appropriately increased or decreased. FIG. 15 also illustrates theoscillating car motor drive SCR of transversing car 56.

Although peen forming is old in the art the use of centrifugal throwingwheels instead of the conventional blast nozzles to accomplish the peenforming is unobvious for a number of reasons. For example it wasbelieved by those skilled in the art that the intensity of a patternproduced by a centrifugal wheel varied across the entire pattern. It hasbeen found, however, that actually the only variance was at the extremesof the pattern. This could be compensated for, however, by providingoverlapping patterns. Additionally, the use of centrifugal wheels has anumber of unexpected advantages. In this respect the wheels are actuallymore efficient than the prior art air nozzles since only about 20 HP isrequired to project the same amount of shot as compared with 420 HP ofan air compressor. The emergence of the demand created by the aircraftindustry thereby created needs for higher and more efficient productionwhereby such performance with prior art nozzles would be totallyunsatisfactory. Furthermore, with the prior art air nozzles in order tochange the velocity of the shot it is necessary to change the pressurewhich is difiicult to accurately adjust because of pressurefluctuations. In contrast with the centrifugal throwing wheels accurateadjustments can be made by simply adjusting the r.p.m.

The concepts of this invention can be applied to flatten a flat sheet bymoving the sheet under a stream of shot; to reshape or apply a simplecontour to a sheet with or without integral stiffening ribs; to formcompound contours with or without stiffeners; to form simple or compoundcontours in a sheet using tooling devices to pre-stress the part in thedesired shape while peening; to flatten or form simple or compoundcontours in the sheet wherein the centrifugal wheels are universallymounted so that the aiming of the shot streams is not restricted; toflatten or form simple or compound contours in a sheet wherein theuniversal wheels are powered for remote actuation of targeting oraiming; and to flatten or form simple or compound contours with theuniversally mounted wheels having any or all of its universal movementspre-programmed by digital, numerical or other controls.

The use of centrifugal wheels for peen forming actually represents asignificant advancement in the art. Heretofore shot peen forming hasbeen treated quite properly as more art than science. Those whodeveloped and practiced it considered the air nozzle a necessary tool ofthe art. With the advent of a centrifugal shot-throwing wheel, peenforming now ceases to be an art and becomes a cost-saving manufacturingmethod of great potential. Some of the user benefits are:

l. Elimination of the length restrictions on long, tapered parts withsimple contours. Such parts are usually formed by bumping on a pressbrake or by forming rolls. Both of these processes impose length limitsbecause of their nature.

2. Elimination of the need for massive stretch-forming dies for shapingsimple contours; also, elimination of the heattreating operation that isalmost always necessary after stretch forming. Parts can be peenformedin the hardened condition or final temper without subsequent heattreatment. Since stretch forming cannot be applied to parts that taperin thickness, this is still another design limitation that can beavoided by using peen forming methods.

3. Improvement in fatigue properties of parts shaped by peen forming.The surface layer of compressive stresses produced by peen forming alsohelps to prevent stress corrosion cracking.

What is claimed is:

l. A method of peen forming metal sheets comprising providingcentrifugal shot throwing wheel means above the sheet, said wheel meansincludes a bank of a plurality of centrifugal throwing wheels, eachwheel of the bank being mounted to a common mounting member with one setof wheels on one side of the mounting member and the remaining set ofwheels on the other side thereof, and said wheels of one set beingstaggered with respect to the wheels in the other set, causing relativelongitudinal movement between the centrifugal shot throwing wheel meansand the sheet so that the sheet has the effect of passing under thecentrifugal shot throwing wheel means, disposing the bank of wheelsacross the sheet in a direction transverse to the direction of relativemovement of the sheet, disposing each centrifugal throwing wheel toproject shot in a pattern extending across the sheet to be formed,maintaining the bank of wheels fixed with respect to the transversedirection, feeding peening shot to the wheel means, projecting thepeening shot from the centrifugal throwing wheels in overlappingpatterns against the sheet, and permanently deforming the sheet with thepeening shot.

2. A method as set forth in claim 1 including disposing a free movingcentrifugal throwing wheel above the sheet in a longitudinal positiondisplaced from the individual wheels of the bank of wheels.

3. A method as set forth in claim 1 including disposing a free movingcentrifugal throwing wheel above the nun sheet downstream from the bankof wheels, and utilizing the free moving centrifugal throwing wheel toapply a touch-up operation to the sheet.

4. A method as set forth in claim 1 wherein said sheet is an aircraftwing skin and is deformed into an air foil contour, including the stepsof masking portions of the skin to create an unmasked area therebetweencorresponding to a dihedral break, disposing the masked skin undercentrifugal throwing wheels, and peening the unmasked dihedral breakarea with shot projected from the centrifugal throwing wheels to form asaddle back integral with the remaining portions of the skin.

5. A method as set forth in claim 4 wherein a pair of the skins areformed with integral saddle backs and the pair of skins are securedtogether to form a wing.

6. A method as set forth in claim 5 wherein prior to the saddle backformation the bank of wheels are rotated and the wheels are oscillatedback and forth across the skin to form the saddle back.

7. A method as set forth in claim 1 wherein the peened surface isblended and smoothed by projecting shot from the bank of wheels at alower intensity than was used in the peen forming operation.

8. A method of saturation peening metal objects comprising providingcentrifugal shot throwing wheel means above the objects, said wheelmeans including a bank of a plurality of centrifugal throwing wheels,said bank being disposed in a longitudinal direction, each centrifugalthrowing wheel being disposed to project shot in a longitudinal patternagainst the objects to be treated, causing relative longitudinalmovement between the centrifugal shot throwing wheel means and theobject so that the objects have the effect of passing under the shotthrowing wheel means, feeding peening shot to the wheel means,projecting the peening shot from the wheel means against the object, andoscillating the wheel means back and forth over the object transverse tothe relative longitudinal movement thereof while the shot is projectedfrom the wheel means.

9. A method as set forth in claim 8 including synchronizing theoscillatory movement of the throwing wheel means with the relativelongitudinal movement to provide a uniform saturation peening.

10. A method as set forth in claim 8 wherein the wheel means includes abank of a plurality of centrifugal throwing wheels, disposing the wheelsin the bank in a staggered manner, creating an enlarged treating areaequal to about the number of wheels plus one times the width of thepattern obtained from a single wheel upon the first pass of the bank ofwheels across the objects with a portion of the enlarged treating areabeing untreated, reversing the direction of the bank of wheels back andforth over the objects for two complete oscillations with an area equalto the enlarged area being peened by at least two of the four passes ofthe two complete oscillations.

11. A method as set forth in claim 10 wherein the bank of wheelsincludes four wheels and the enlarged area is five times the areacovered by a pattern produced by one of the wheels individually.

12. A method as set forth in claim 8 including mounting the individualwheels of the bank to a common mounting member with one set of wheels onone side of the mounting member and the remaining sets of wheels on theother side thereof, and staggering the wheels of one set with respect tothe wheels in the other set.

i i l

1. A method of peen forming metal sheets comprising providing centrifugal shot throwing wheel means above the sheet, said wheel means includes a bank of a plurality of centrifugal throwing wheels, each wheel of the bank being mounted to a common mounting member with one set of wheels on one side of the mounting member and the remaining set of wheels on the other side thereof, and said wheels of one set being staggered with respect to the wheels in the other set, causing relative longitudinal movement between the centrifugal shot throwing wheel means and the sheet so that the sheet has the effect of passing under the centrifugal shot throwing wheel means, disposing the bank of wheels across the sheet in a direction transverse to the direction of relative movement of the sheet, disposing each centrifugal throwing wheel to project shot in a pattern extending across the sheet to be formed, maintaining the bank of wheels fixed with respect to the transverse direction, feeding peening shot to the wheel means, projecting the peening shot from the centrifugal throwing wheels in overlapping patterns against the sheet, and permanently deforming the sheet with the peening shot.
 2. A method as set forth in claim 1 including disposing a free moving centrifugal throwing wheel above the sheet in a longitudinal position displaced from the individual wheels of the bank of wheels.
 3. A method as set forth in claim 1 including disposing a free moving centrifugal throwing wheel above the sheet downstream from the bank of wheels, and utilizing the free moving centrifugal throwing wheel to apply a touch-up operation to the sheet.
 4. A method as set forth in claim 1 wherein said sheet is an aircraft wing skin and is deformed into an air foil contour, including the steps of masking portions of the skin to create an unmasked area therebetween corresponding to a dihedral break, disposing the masked skin under centrifugal throwing wheels, and peening the unmasked dihedral break area with shot projected from the centrifugal throwing wheels to form a saddle back integral with the remaining portions of the skin.
 5. A method as set forth in claim 4 wherein a pair of the skins are formed with integral saddle backs and the pair of skins are secured together to form a wing.
 6. A method as set forth in claim 5 wherein prior to the saddle back formation the bank of wheels are rotated 90* and the wheels are oscillated back and forth across tHe skin to form the saddle back.
 7. A method as set forth in claim 1 wherein the peened surface is blended and smoothed by projecting shot from the bank of wheels at a lower intensity than was used in the peen forming operation.
 8. A method of saturation peening metal objects comprising providing centrifugal shot throwing wheel means above the objects, said wheel means including a bank of a plurality of centrifugal throwing wheels, said bank being disposed in a longitudinal direction, each centrifugal throwing wheel being disposed to project shot in a longitudinal pattern against the objects to be treated, causing relative longitudinal movement between the centrifugal shot throwing wheel means and the object so that the objects have the effect of passing under the shot throwing wheel means, feeding peening shot to the wheel means, projecting the peening shot from the wheel means against the object, and oscillating the wheel means back and forth over the object transverse to the relative longitudinal movement thereof while the shot is projected from the wheel means.
 9. A method as set forth in claim 8 including synchronizing the oscillatory movement of the throwing wheel means with the relative longitudinal movement to provide a uniform saturation peening.
 10. A method as set forth in claim 8 wherein the wheel means includes a bank of a plurality of centrifugal throwing wheels, disposing the wheels in the bank in a staggered manner, creating an enlarged treating area equal to about the number of wheels plus one times the width of the pattern obtained from a single wheel upon the first pass of the bank of wheels across the objects with a portion of the enlarged treating area being untreated, reversing the direction of the bank of wheels back and forth over the objects for two complete oscillations with an area equal to the enlarged area being peened by at least two of the four passes of the two complete oscillations.
 11. A method as set forth in claim 10 wherein the bank of wheels includes four wheels and the enlarged area is five times the area covered by a pattern produced by one of the wheels individually.
 12. A method as set forth in claim 8 including mounting the individual wheels of the bank to a common mounting member with one set of wheels on one side of the mounting member and the remaining sets of wheels on the other side thereof, and staggering the wheels of one set with respect to the wheels in the other set. 